Reduction of the viscosity of cellulose nitrate



Patented Feb. 6, 1934 REDUCTION OF THE VISCOSITY OF CELLULOSE NITRATE Roderick K. Eskew, Rahway, N. J., assignor to Dupont Viseoloid Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application March 25, 1932 Serial No. 601,302

8 Claims.

This invention relates to the purification, bleaching, and more particularly to the viscosity reduction of cellulose nitrate; and especially to these operations as carried out on nitrocellulose "below 12.5% N2 content, as used in the lacquer and pyroxylin plastics industries, although it is not confined to the treatment of nitrocellulose of a nitrogen content below 12.5%.

It is well-known that nitrocellulose of approxi- 0' mately 12% nitrogen content and low viscosity is extensively used in lacquers and enamels for furniture and automobiles. This is possible because the property of low viscosity of the nitrocellulose permits it to be used in making solu- ;tions of high concentration but still sufficiently fluid to be sprayed or brushed. It is also true that the property of low viscosity in the case of nitrocellulose of approximately 11% nitrogen content is of importance in camphor-alcohol plastics to be used in such a way that the flowability of the plastic is important, as in the extrusion of rods and tubes.

The prior art of purification of nitrocellulose involved the boiling of the nitrocellulose in dilute spent nitrating acid, washing out the acid, and then bleaching if specially good color were desired. Low viscosity nitrocellulose has been made by nitrating at high temperatures with attendant low yields and high acid losses, or as an alternative, digesting the nitrocellulose under pressure.

I have discovered a safe, rapid and economical method of accomplishing nitrocellulose viscosity reduction without the necessity of working under pressure. The method, if desired, can be substi tuted, in part at least, for the usual purification boil or steep which constitutes a part of the process of nitrocellulose manufacture.

One object of the invention is to provide a method of the above mentioned, and other, desirable characteristics. A further object is to provide a method for reducing the viscosity of cellulose nitrate, which method embodies the advantages of working with pyridine as the viscosity reducing agent without the disadvantages incident to the pyridine being retained by the ester; certain of such disadvantages being instability and spontaneous decomposition of the ester when in the undissolved state, and the progressive reduction of viscosity of solutions of the ester. To these ends, and also to improve generally upon methods of the general character indicated, the invention consists in the various matters hereinafter described and claimed.

In general accordance with the invention, nitrocellulose, preferably in the undissolved state,

is treated with a solution of pyridine bases the desired reduction of the viscosity has been effected, and then the nitrocellulose is treated with an aldehyde, e. g. formaldehyde or acetaldehyde. I have discovered that the aldehydes, even in the absence of a penetrant, are peculiarly effective agents for removing pyridine from nitrocellulose, and the invention embodies this discovery. The product obtained is of good fiber length so that the mechanical losses that may later occurbe- 66 cause of the particular viscosity reduction employed for the nitrocellulose as in the present case are particularly small. Furthermore, the product is stable in a colloided or uncolloided state, possessing in that respect an essential prop- 70 erty heretofore unobtainable when it has been attempted to treat nitrocellulose with pyridine to reduce the viscosity characteristics thereof.

Prior to the treatment with the aldehyde, it is desirable to wash the nitrocellulose. In treating with the aldehyde, it is desirable (although not essential) to use an acid as I have found that it aids the pyridine removal and lightens the color of the nitrocellulose so that, if the nitrocellulose is to be bleached, a milder bleach can be used. After the aldehyde treatment the nitrocellulose is preferably washed, desirably with hot water. Even without a subsequent bleaching, and particularly when an acid is used with the aldehyde, a product of reasonablygood color is obtained and one which is suitable for. use in cases where entire freedom fromcolor is not essential.

When especially good color is desired, the nitrocellulose may, after the removal of the pyridine by the aldehyde treatment, be bleached with any of the usual bleaching agents such as hypochlorite according to the ordinary procedure in the industry. This bleach given after the'aldehyde treatment is not a treatment to remove pyridine, since a chlorinating agent under ordinary circumstances does not effectively remove pyridine from nitrocellulose in the fibrous or uncolloided state, but is for the sole purpose ofde-1 stroying the yellow color arising from degradation products of the viscosity reducing action. While I do not base the invention on, or confine it to any particular explanation, it appears to me probable that the effectiveness of aldehydes as agents for removing pyridine from nitrocellulosic materials arises from the characteristic reactivity of the V I H-:=o group. It is known that they have the property hours, and then is washed.

and boiledfor about six hours.

of combining with a great variety of substances and in their present application they probably form an addition compound with the pyridine. In the case of formaldehyde the compound is probably C5I-I5N.HCHO or an unstable alkine C5H4N.CII2OH (E. Formanek, Berichte 1905, 38, p. 94445). It is readily soluble in hot water and is therefore removed from the nitrocellulose by washing subsequent to the aldehyde treatment.

In specific illustration of the invention, but without limitation of the invention thereto, the procedure may be as follows:

The nitrocellulose (in this instance previously stabilized) is heated in the undissolved state with a solution of pyridine or pyridine bases in any desired way, to reduce the viscosity to the desired amount, say to a viscosity of second (3 seconds in a 20% solution). The pyridine solution isthen drained off, the nitrocellulose is washed and then covered with 20 times its weight of water containing of formaldehyde, the mass is boiled at atmospheric pressure for 3 (Six washes should generally be adequate.)

In treating a nitrocellulose only partially staibilized by the usual dilute acid boil, and having, say, a stability of approximately 12 minutes by the methyl violet papertest at 134.5" 0., the nitrocellulose, washed free from mechanically entrained acid, is treated in the undissolved state with pyridine solution, in any desired way to reduce its viscosity, the pyridine solution drained off, and the nitrocellulose covered with 20 times its weight or" water containing formaldehyde. Inconsideration of the but partial stabilization of the nitrocellulose approximately 2% mixed acid (approximately 20% HNOs, 6)%' H2894, 20% B20) is also added. The whole is then boiled for 6 hours at the end of which time the pyroxylin is washedacid free. It will then f be stabilized by virtue of the acid boil and be free from pyridine as the result of the formaldehyde treatment.

i "The sameas Example II except after the pyridine solution is drained 01f, the nitrocellulose is covered with 20 times its weight of water containing formaldehyde and boiled for three hours. At the end of this time the formalde- "hyde bath is drained off and the pyroxylin again covered with water containing about 2% mixed acid and then boiled about six hours to complete the stabilization. Thepyroxylin iswashed acid free in the usual manner.

Same as Example II except after draining off the water containing pyridine, the nitrocellulose is covered with water containing .2% mixed acid The water is thendrained off, the pyroxylin covered with about20 times its weight of water containing formaldehyde and boiled for three hours. At the end of this treatment the pyroxylin is washed free from traces of formaldehyde in the usual manner.

While it is not necessary for the effectiveness of the pyridine removal treatment per se to add acid during that treatment, it is sometimes desirable even in the case of treating'stabilized nitrocellulose to add acid in order to neutralize any excess pyridine in the water entrained in the nitrocellulose, rather than using up aldehyde for this purpose. Also, the use of an acid with the aldehyde is advisable in some cases since it partially destroys any color developed by the viscosity reducing treatment and thereby reduces the strength of bleach which may be used subsequently.

Although I have particularly referred to pyridine as the viscosity reducing agent, removable by the present treatment it should be understood that the treatment is applicable also to the homologues of pyridine, e. g. the picolines, lutidines and collidines and the analogous compound quino-line, all of which occur in bone-oil and all s of which act in the same general way upon nitrocellulose. Furtherrnore, although I have referred particularly to formaldehyde as the pyridine destroying agent it will be understood that other aldehydes e. g. acetaldehyde as well as the higher homologues of these substances; and benzaldehyde, would function in ,a similar way. I also mean to include these compounds in the polymerized state in which they frequent- 1y occur, e. g. paraformaldehyde and paraldehyde, since they function as aldehydes in the presence of dilute acid. Formic acid may also be used as it acts both as an aldehyde and as an acid.

As illustrating methods of reducing viscosity with pyridine the following may be cited:

(a) Stable nitrocellulose (N2 content 12.44% and a viscosity of 15 seconds) is steeped at 85 C. to C. in 26 times its Weight of water containing 2% by volume of C. P. pyridine. The resulting viscosity will be -3 seconds at the end of approximately 4% hours, 2 seconds at the endin it except that a temperature of 100 C. is

used and a mixture of pyridine bases boiling at 140 C. (such as used in denaturing alcohol) is substituted for the C. P. pyridine, a viscosity of A2 second (3 seconds in 20% solution) will be obtained in approximately 9 hours.

Procedure b in comparison with a shows the increased rate of viscosity reduction resulting from the use of a higher concentration of pyridine. Procedure 0 illustrates how a mixture of pyridine bases may be effectively and even advantageously substituted for C. P. pyridine as such mixtures boil higher than pyridine and can therefore be used at higher operating temperatures. r

In regard to the effectiveness of the aldehyde treatment in eliminating pyridine, the following experiment is of interest:

A sample of stable nitrocellulose having a viscosity of /2 second (.3 seconds in 20% solution) was used. This was impregnated with pyridine by immersing it at room temperature for hour in 20 times its weight of water containing 2% C. P. pyridine. The nitrocellulose was then drained, acidified with dilute H2504, washed acid free and divided into three parts, A, B, and C, and these parts diiferently treated as f0110WSI- Part A. Washed 6 times .with H2O with wringand boiled 9 hours.

ing of the nitrocellulose between each wash.

Part B. Bleached at C. for 1 hour in a bath containing enough NaOCl to give .1% available C12 on the liquor basis, and then acidified with .1% oxalic acid as an antichlor and kept another hour at 50 C. The pyroxylin was then washed acid-free.

Part C. Boiled 3 hours in a bath containing /z% formaldehyde and /2% mixed acid, then washed acid-free.

The nitrocellulose samples from A, B and C were then dehydrated in the usual manner and a 20% solution prepared of each sample, and also a like solution of a sample of the nitrocellulose untreated with pyridine. All the solutions were then aged at 45 C. for 10 days with periodic viscosity determinations.

Since it is known that minute traces of pyridine in a nitrocellulose solution cause a decided and progressive drop in viscosity on ageing, which drop is greatly accelerated by increased temperatures, the rate of drop of the viscosity of these solutions compared with a standard using well stabilized nitrocellulose which has had no contact with pyridine gives an excellent indication of the completeness of removal of the pyridine from the nitrocellulose. When the time of ageing the solution is plotted on an arithmetic scale against the viscosity on a logarithmic scale, the result is a straight line. This indicates that the rate of drop in viscosity is constant over the period under consideration, 1. e. ten days. The drop in viscosity in any hour expressed as percent of the viscosity at the beginning of that hour may be termed the rate of drop, and since as shown this rate is the same for a given sample at any period of the test, a comparison ofthe rates of drop of two samples may be properly considered as representing the comparative effectiveness of the pyridine-removing treatments under consideration.

It was found that the rate of drop in the case of samples A and B was identical (.07), while the rate in the case of C was .03, less than half as much. The standard nitrocellulose which had no pyridine treatment had a rate of .03. This clearly indicates the ineffectiveness of mere washing or a chlorinating treatment for removing pyridine from nitrocellulose and shows also that cornplete removal can be accomplished by a combined acid and aldehyde treatment.

Also of interest as illustrating the effectiveness of the aldehyde treatment is the following':--In applying the invention to a nitrocellulose only partially stabilized by the usual dilute acid boil, and having, say, a stability of approximately 12 minutes by the methyl volet paper test at 134.5 (3., the nitrocellulose, washed free from mechanically entrained acid, was covered with 20 times its weight of water containing 2% by volume of pyridine bases (boiling point 140 C.) The nitrocellulose was then drained and again covered with 20 times its weight of water containing 2% mixed acid and the whole boiled for six hours to complete stabilization. This was divided into three parts D, E, and F. and these were dfferently treated as follows:-

Part D. No further treatment.

Part E. Boiled 3 hours in formaldehyde plus /{2% mixed acid and then washed acid-free.

Part F. Boiled 3 hours in acetaldehyde and given 6 washes.

(Before the division of the nitrocellulose it was, of course, Washed free of acid after the acid boil.)

When solutions of these samples were subjected to an ageing test at 45 C. as described in connection with the beforementioned experiments, it was found that the rate of drop was .09 for sample D, .03 for sample E, and .04 for sample F. Nitrocellulose of approximately the same viscosity which had not been treated with pyridine had a rate of .03 and a sample which had been reduced in viscosity by the pyridine treatment and had then been given a thorough washing with water had a rate of .19. Thus, the aldehyde-treated portions had no significantly greater drop than samples that had had no pyridine treatment, showing that the aldehyde treatment had been wholly effective in removing pyridine. On the other hand, pyridine-treated samples that had been washed thoroughly, or had been given a 6 hour acid boil only had a very rapid drop showing that these two methods of treatment are ineffective in removing pyridine.

In reference to the viscosity referred to herein, they were determined by the method most generally known in the industry, i. e. 12.2% of dry nitrocellulose were dissolved in a solvent consisting of percent by volume, ethyl alcohol 25, ethyl acetate 20, toluol 55. The viscosity was taken as the time in seconds required for a inch diameter steel ball to fall through 10 inches of this solution at 25 C. in a tube 14 long and 1 inside diameter. However, for viscosities at and under 1 second as determined by this method, additional determinations were made using a 20% solution of the nitrocellulose in a solvent such as just mentioned, and measuring viscosity by the time required for the solution to run from a specially constructed pipette. This was done to magnify differences in order to obtain a more accurate measure of the rate of drop.

In cases wherein the present treatment is to be applied to already fully stabilized nitrocellulose, e. g. Example I above, the stabilization may of course be done in any of the various ways of the art for stabilization of particular. types of nitrocellulose. In cases wherein stabilization is to be carried out subsequently to the pyridine treatment, it is advantageous to perform the pyridine removal treatment and the stabilization simultaneously, this being illustrated by Examples II, III and IV above.

It will be understood that in Examples III and IV, the procedures outlined .are to take the place of the various steps of Example II after the draining off of the pyridine solution, rather than as additional procedures introduced into Example II. In regard to the pyridine treatment referred to in Examples I and II (and III and IV) this treatment may, for example, be in accordance with a, b, or 0 above, these treatments being applicable to reducing the viscosity of both stable and partially stabilized nitrocellulose.

I claim:

1. A process for reducing the viscosity of cellulose nitrate which comprises acting thereon with a pyridine base as a viscosity reducing agent, and thereafter treating the cellulose nitrate with an aldehyde to remove the said agent.

2. A process for reducing the viscosity of cellulose nitrate which comprises acting thereon with pyridine, and thereafter treating the cellulose nitrate with formaldehyde to remove the pyridine.

3. A process for reducing the viscosity of cellulose nitrate which comprises acting thereon with pyridine, and thereafter treating the cellulose nitrate with acetaldehyde to remove the pyridine.

4. A process for reducing the viscosity cellulose nitrate which comprises acting thereon with a pyridine base as a viscosity reducing agent, and following the same with a step of treating with an aldehyde and a step of treating with water, one of said steps being performed in the presence of a small amount of acid.

5. A process for reducing the viscosity of cellulose nitrate which comprises acting thereon with a pyridine base as a viscosity reducing agent and thereafter treating the cellulose nitrate with an aldehyde solution containing a small amount of acid.

6. A process for reducing the viscosity of cellulose nitrate Which comprises, acting on the cellulose nitrate with a pyridine base as a viscosity reducing agent, treating the cellulose nitrate with an aldehyde to remove said agent, and treating the cellulose nitrate in water containing a small amount of acid.

7. A process for reducing the viscosity of cellulose nitrate which comprises, acting on the cellulose nitrate with a pyridine base as a viscosity reducing agent, treating the cellulose nitrate in water containing a small amount of acid, and

treating the cellulose nitrate with an aldehyde to 

